text: Hundreds of diatomic molecules have been identified in the environment of the Earth, in the laboratory, and in interstellar space. About 99% of the Earths atmosphere is composed of two species of diatomic molecules: nitrogen (78%) and oxygen (21%). The natural abundance of hydrogen (H) in the Earths atmosphere is only of the order of parts per million, but H is the most abundant diatomic molecule in the universe. The interstellar medium is dominated by hydrogen atoms.